1. Field of the Invention
The invention relates to an gas laser pumped by an electrical gas discharge, and particularly to a corona-type preionization device and technique for generating a stable pulsed gas discharge for pumping of an active medium of an excimer or molecular fluorine gas discharge laser.
2. Discussion of the Related Art
UV-preionization of an electrical discharge in a pulsed gas laser is typically realized by means of an array of spark gaps or by another source of UV-radiation (surface, barrier or corona gas discharges), disposed in the vicinity of at least one of the solid electrodes of the main discharge of the laser. Early on in the development of excimer lasers (e.g., KrF, ArF, XeCl, XeF, etc.), conventional pulsed electrical gas discharges typically used for pumping the active media exhibited a high degree of instability. The development of discharge instabilities causes the glow discharge, a precondition for laser emission, to have a short phase (e.g., having a typical duration from 10-100 ns) and to thus be terminated more quickly than is preferred. The desired way of generating a high quality gas discharge for use in excimer and molecular lasers, including the molecular fluorine (F2) laser, is to provide an intense, yet uniform preionization of the gas volume before the main gas discharge occurs.
One way of providing this preionization is by photo-ionizing the laser gas with UV-light emitted from an auxiliary gas discharge before the main gas discharge is switched on. Some known methods of preionizing high pressure gas lasers include x-ray, spark and corona-gap preionization. See R. S. Taylor and K. E. Leopold, Pre-preionization of a Long Optical Pulse Magnetic-Spiker Sustainer XeCl Laser, Rev. Sci. Instum. 65 (12), (December 1994). The spark method involves the use of spark gaps (ordinary or stabilized by a dielectric surface; see, e.g., U.S. patent application Ser. No. 09/532,276 which is assigned to the same assignee as the present application and is hereby incorporated by reference). The corona-gap method involves the use of pulsed corona-like discharges near a dielectric surface. Spark gap preionizers produce a periodic series of preionized volumes of laser gas along the elongated discharge area of the laser, resulting in some nonuniformity of the discharge. Thus, corona-type preionization is preferred in the present invention.
Areas of focus for design improvement of corona-gap preionizers include the geometry of the dielectric body, and the arrangement of the preionization electrodes. See U.S. Pat. No. 4,718,072 to Marchetti et al. (showing a grounded internal preionization electrode surrounded by a dielectric having a positive potential applied to its outer surface through contact with the positively biased main electrode); European Patent Application (published) EP 0 532 751 A1 (showing an internal preionization electrode surrounded by a dielectric buried in one of the main electrodes); U.S. Pat. No. 4,953,174 to Eldridge et al. (showing the dielectric surrounding an internal preionization electrode abutting with a main discharge electrode); see also R. Marchetti et al., A New Type of Corona-Discharge Preionization Source for Gas Lasers, J. Appl. Phys. 56 (11), (Dec. 1, 1984); U.S. Pat. No. 4,380,079 to Cohn et al.
Reconfiguration of external, electrical circuits is another area where corona-gap pre-ionizer design improvement efforts have been focused. See Taylor et al., citation above; U.S. Pat. No. 5,247,531 to Muller-Horsche (showing an excitation of preionization electrodes affected by the same high voltage source as the main discharge electrodes including a time delay inductance between them), U.S. Pat. No. 5,247,534 to Muller-Horsche (including flow bodies configured to facilitate laser gas flow and formed of material exhibiting secondary x-ray emission characteristics) and U.S. Pat. No. 5,247,535 to Muller-Horsche (disclosing electron emission from a heated cathode, wherein x-rays emitted as the electrons impinge upon a separate anode serve to preionize the laser gas volume).
U.S. Pat. No. 5,337,330 to Larson, hereinafter referred to as the ""330 patent, describes the conventional corona-like preionization arrangement generally shown in FIG. 1a. See also U.S. Pat. No. 5,247,391 to Gormley, and U.S. Pat. No. 4,953,174 to Eldridge et al. A discharge chamber having the preionization arrangement of FIG. 1a includes a high voltage main electrode 1 and a grounded main electrode 2. Each preionization unit includes one internal preionization electrode 3a located on one side of main discharge region 5 between the main discharge electrodes 1,2. Each preionization unit includes a dielectric tube 3b of generally cylindrical shape surrounding the internal preionization electrode 3a. A preionization discharge (ultraviolet emission) 4 from the preionization electrodes 3a and 6 and dielectric tubes 3b causes a preionization of the volume of the main gas discharge. A pair of external preionization electrodes 6 of the preionization units comprise metal plates and are each directly connected to the nearby main discharge electrode 1 (e.g., the cathode at high potential). FIG. 1b shows a conventional preionization unit setup wherein only one internal corona-discharge preionization electrode 3a is employed. See U.S. Pat. No. 4,240,044 to Fahlen et al.
In the case of the preionization unit of either of FIG. 1a or 1b, energy stored in the dielectric tubes 3b during a preionization phase, will also be absorbed into the main discharge 5. However, that added energy typically will not increase the laser output due to a high wave impedance of the dielectric tubes 3b. The tubes 3b act much like a charged transmission line in that this wave impedance is typically much higher than the impedance of the main gas discharge. The high wave impedance is caused by a distributed inductivity of each whole dielectric tube 3b (as a transmission line) and a concentrated inductivity at the point of electrical connection of the tubes 3b with the internal corona discharge electrodes 3a. 
The residual energy produces high voltage electrical oscillations between the capacitance of the dielectric tubes 3b of the preionization units and the main gas discharge volume. These high voltage oscillations are undesirable because they significantly reduce the ability of the dielectric tubes 3b of the preionization unit to resist direct high voltage breakdown and over-flashing near the open ends of the dielectric tubes 3b. Moreover, these oscillations deteriorate the quality of the main gas discharge 5 and thus hinder the operation of the laser, particularly during operation at a high repetition rate. Furthermore, the oscillations cause additional wear to the main gas discharge electrodes 1,2 and the internal corona discharge electrodes 3a, and also cause contamination and a reduced lifetime of the laser system.
FIG. 1c shows one technique described in the ""330 patent for alleviating the high-voltage breakdown and over-flashing problems caused by these oscillations. That technique involves providing a preionization tube 7 with bushings 8 at opposite ends made from an identical material as the tube 7 and integral with the tube 7. The tube 7 with the opposed bushings 8 is described as being machined from a single integral piece of material. U.S. Pat. Nos. 5,818,865 and 5,991,324 describe furtherances of the design described in the ""330 patent. The manufacturing of the tubes described in the ""330, ""865 and ""324 patents undesirably involves complexity and cost. Moreover, the high voltage oscillations continue to degrade the quality of the discharge and produce undesirable wear to the main gas discharge electrodes and the internal corona discharge electrodes, and also cause contamination and a reduced lifetime of the laser system, as discussed above.
FIGS. 1d and 1e illustrate another technique which is described at U.S. patent application Ser. No. 09/247,887, hereinafter referred to as the ""887 application, which is assigned to the same assignee as the present application, and is hereby incorporated by reference into the present application. In the ""887 application, a preionization tube 9 is provided as shown in FIG. 1d with a sealed end 10 and a thick, open end 11 to address the breakdown and over-flashing problems discussed above at each end. In addition, the ""887 application discloses to connect the internal preionization electrode as shown in FIG. 1e to ground and/or an electrical circuit 13 including active or passive electrical components via an electrical feedthrough 14 to the outside of the discharge chamber. The preferred electrical circuit disclosed in the ""887 application includes a resistor connected to ground and having a resistance comparable to or greater than the wave impedance of the oscillating contour of the preionization unit. The connection to ground via the resistor of the internal preionization electrode serves to dampen the strength of the oscillations.
It is an object of the invention to design a preionization unit for a laser having a high quality gas discharge by providing an intense, yet uniform, preionization of the gas volume between the main discharge electrodes.
It is also an object of the invention to provide a preionization unit wherein such that over-flashing and high voltage breakdown at the ends of the dielectric tube is prevented.
It is another object of the invention to prevent electrical oscillations from arising out of residual energies stored in the dielectric tube.
The present invention meets all of these objects and addresses the shortcomings of conventional preionization techniques by providing a preionization assembly including a preionization unit for a gas laser which comprises an internal preionization electrode having a dielectric tube around it. The internal electrode is connected to an advantageous electrical circuit. This connection to the electrical circuit reduces the voltage across the dielectric tube, while permitting sufficient preionization of the laser gas. The reduced voltage also allows the dielectric tube to have a uniform bushingless design, and to comprise a standard purity material, without having the flashing-over observed in conventional systems wherein the preionization voltage is not reduced.
Preferably, the internal electrode is connected via a feedthrough to the circuit, which is external to the discharge chamber. Whether or not the circuit is external to the discharge chamber, the circuit preferably includes a resistive element connected in parallel with either a capacitive element or a series combination of a capacitive element and a resistive element. The circuit connection permits the tube to meet the above objects while having a simple manufacturing design comprising an ordinary ceramic with relaxed purity and density specifications relative to conventional tubes, although high purity ceramics may be used. The tube preferably has a substantially cylindrical shape and a uniform diameter, and is preferably open at both ends. The tube further preferably comprises polycrystalline alumina (Al2O3), and may comprise sapphire. The preionization assembly may comprise one or two or more tubes.
An excimer or molecular fluorine laser system is also provided, including a discharge chamber containing a gas mixture, a first and a second spaced apart main discharge electrodes having a discharge area therebetween within the discharge chamber, the first main discharge electrode being maintained at ground potential, a pulsed power supply unit for applying electrical pulses to the second main discharge electrode for producing discharges between the first and second main discharge electrodes for energizing the gas mixture, at least one preionization unit disposed within the discharge chamber for preionizing the gas mixture within the discharge area prior to application of the electrical pulses to the main discharge electrodes, the preionization unit including first and second preionization electrodes having a dielectric therebetween, an electrical circuit coupled between the first preionization electrode and ground potential, said second preionization electrode being connected to said first main discharge electrode, such that a potential difference applied across the first and second preionization electrodes is substantially less than is applied across the first and second main discharge electrodes by the electrical pulses supplied by the pulsed power supply unit, and a resonator for generating a laser beam, wherein the reduced potential difference applied across the first and second preionization electrodes provides reduced over-flashing and electrical oscillations from residual energies stored in the dielectric.